1. Field of the Invention
This invention relates generally to heaters. More specifically, the present invention relates to a linear heater which can be advantageously used in an office automation apparatus such as a photocopier or electrophotographic printer for fixing images on a paper sheet for example. The present invention also relates to a method for adjusting the resistance of such a heater.
2. Description of the Prior Art
Various types of linear heaters are known for fixing images (deposited toner) on a paper sheet in photocopiers or electrophotographic printers (e.g. laser beam printer). Typical examples include a lamp heater and a roller heater.
However, the lamp heater and roller heater are equally disadvantageous in that there is a limitation in reducing size (thickness) and cost. Further, the lamp heater is easily damaged due to the nature of material, whereas the roller heater has a complicated structure due to the necessity of incorporating plural heating elements within the roller.
To eliminate the problems of the conventional heaters, it has been proposed to use a strip heater for fixing images on a paper sheet in electrophotography, as disclosed in U.S. Pat. No. 5,068,517 for example. Specifically, the strip heater disclosed in this U.S. patent comprises an elongate insulating substrate having a surface formed with a printed resistor strip. Each end of the resistor strip is connected to an enlarged terminal electrode made of silver for connection to a power source. The resistor strip, which is made of e.g. silver-palladium alloy, generates heat when a current is passed therethrough.
Obviously, the prior art strip heater is very simple in arrangement. Further, the strip heater can be made very thin and light by reducing the thickness of the substrate. Moreover, the strip heater is also advantageous in that the time required for warming up is very short. However, the prior art strip heater still has the following problem.
According to the prior art, a resistor strip is formed by printing a resistor material paste (e.g. silver-palladium) on a substrate and baking the printed paste for fixation. Obviously, it is necessary to print a resistor material paste and bake the printed paste equally (uniformly) for all heaters of the same specification, thereby insuring a uniform quality.
However, the composition and viscosity of the resistor material paste tend to vary inevitably upon every change from one lot (batch) to another. Further, it is difficult to print the material paste with uniform thickness and width for all heaters, as also is to realize strictly uniform baking conditions. Thus, different heaters of the same specification tend to have different heating characteristics.